In recent years, a considerable interest in silica reinforced rubber articles, such as tires, has developed. For instance, U.S. Pat. No. 5,227,425 indicates that tires made with tread formulations that contain silica exhibit a number of important performance advantages over tires manufactured using carbon black as the filler. The utilization of silica in tire tread formulations is believed to (a) lower rolling resistance, (b) provide better traction on snow, and (c) lower noise generation, when compared with conventional tires filled with carbon black.
U.S. Pat. No. 5,227,425 discloses a sulfur-vulcanizable rubber composition obtained by thermomechanical work of a conjugated diene compound and an aromatic vinyl compound prepared by solution polymerization in a hydrocarbon solvent having a total content of aromatic vinyl compound of between 5% and 50% and a glass transition temperature (Tg) of between 0° and −80° C. with 30 to 150 parts by weight per 100 parts by weight of elastomer of a silica having a BET surface area of between 100 and 250 m2/g, a CTAB surface area of between 100 and 250 m2/g, an oil absorption measured in DBP of between 150 and 250 ml/100 g, and an average projected area of the aggregates greater than 8500 nm2 before use and between 7000 and 8400 nm2 after thermomechanical mixing as well as the additives conventionally employed, with the exception of the sulfur vulcanization system, comprising at least one heat step reaching a temperature of between 130° C. and 180° C. for a suitable period of time of between 10 seconds and 20 minutes which is a function of the temperature selected in order to carry out the mechanical work and of the nature and volume of the components subjected to the mechanical work, followed by a finishing step consisting of the incorporating of the vulcanization system by mechanical work at a temperature below the vulcanization temperature.
Sometimes rubber for tires is supplied by a rubber producer to a tire manufacturer in the form of a masterbatch containing an elastomer, an oil extender, and a filler. The traditional filler has been carbon black in the form of fine particles. These particles have hydrophobic surface characteristics and will therefore disperse easily within the hydrophobic elastomer. In contrast, silica has a very hydrophilic surface and considerable difficulty has been encountered in dispersing silica in the hydrophobic rubbery elastomer.
A number of techniques have been developed to incorporate such reinforcing agents and fillers into the polymer compositions, including both wet and dry blending processes. The incorporation of silica and carbon black as reinforcing agents and/or fillers into rubbery elastomers is far more complex than one might initially believe. One problem in wet blending of silica with latices of such polymers arises from the fact that the hydrophilic silica has a tendency to associate with the aqueous phase and not blend uniformly with the hydrophobic polymer.
To overcome the problems associated with the hydrophilic nature of the silica, U.S. Pat. No. 3,317,458 proposed a process whereby an aqueous solution of silicic acid was treated so as to precipitate silica directly onto the surface of an aqueous carbon black in paste form. One significant limitation of this technique is that the surface of the carbon black is altered thus obviating the utility of the many surface structure characteristics of specialty carbon blacks available to the skilled compounder in designing filled, reinforced polymers for specific end uses.
Attempts have been made to use cationic emulsifiers in an effort to distribute such fillers and/or reinforcing agents in polymeric latices; notably among them are quaternary alkylated ammonium halides (see German Patent No. 1,188,797). It has been found, however, that such quaternary ammonium compounds have a tendency to react with the carbon black, dispersing it into the aqueous phase; that limiting the ability to blend carbon black with the polymeric components in the latex. In U.S. Pat. No. 3,686,113 aqueously wet silica is treated with oleophilic quaternary ammonium compounds to blend with an aqueous dispersion of an elastomer containing an anionic dispersing agent. In addition to the problem referenced above with carbon black dispersion, unfortunately, such techniques require specific coagulating agents.
Perhaps the most commonly employed practice, used commercially, is the technique of dry blending either silica, carbon black or both of them into rubber and thermoplastic polymers in a high-shear milling operation. That practice has many limitations. Notable among them include the tendency of the filler particles to agglomerate to each other, resulting in nonuniform dispersion of the filler throughout the polymer constituting the continuous phase. Another problem commonly experienced in such high-shear milling operations is the tendency of the polymers to degrade during milling. That necessitates the use of higher molecular weight polymers, which sometimes require the incorporation of various types of processing aids to facilitate mixing and dispersion of the filler particles into the polymer constituting the continuous phase. The cost associated with the use of such processing aids also increases the manufacturing cost of the polymeric compound or article and can lead to diminished product performance. The use of processing aids has the further disadvantage in that such processing aids may have a negative effect on the cure or end use of the polymer. Such dry blending techniques also result in additional processing costs, in part due to excessive equipment wear caused by the abrasive fillers.
To improve dispersion of the silica during dry mixing, it has been proposed that such compounding operations employ a silica which has been treated with an organosilane coupling agent having dual functionality. Representative of such coupling agents are those well known compounds that include both an organic groups, such as an amino alkyl group, a mercaptoalkyl group, or a polysulfidic-bis-organo alkoxy silane group bonded directly to the silicon atom along with a readily hydrolyzable group, such as an alkoxy group as represented by a methoxy group or an ethoxy group, likewise bonded directly to the silicon atom. In those systems, it is generally recognized that the alkoxy group hydrolyzes in the presence of moisture typically found on the surface of the silica to form the corresponding silanol which reacts with or condenses in the presence of the silica surface to bond the silicon atom to the silica surface. The organic groups likewise attached to the silicon atom are thus available for chemical reaction with the polymer matrix during vulcanization. As a result, the polymer matrix may become chemically bonded by means of the coupling agent to the silica surface during cure or vulcanization of the polymer. Problems associated with the use of such silanes during compounding are unpleasant odors, premature curing, and/or scorching.
In an effort to overcome the problems associated with the use of silane coupling agents, it has been proposed in U.S. Pat. No. 5,405,897 to employ phenoxy acidic acid along with a methylene donor in place of the conventional organosilanes. The foregoing patent suggests that the use of such a system provides improved physical properties and reduced viscosity of the melt during compounding.
Various other attempts have been made to overcome the problems associated with wet blending such fillers and/or reinforcing agents with polymer latices. For example, it has been proposed, as described in U.S. Pat. No. 3,055,956 and U.S. Pat. No. 3,767,605 to add carbon black in the form of a slurry directly to an emulsion polymerization process of rubbery polymer, at the latex stage, followed by coagulation and recovery of a rubber-carbon black masterbatch. Such processes work well with carbon black, but fail to incorporate substantial amounts of fine particulate silica. U.S. Pat. No. 4,481,329 proposes a process for dispersing carbon black and like fillers into concentrated rubber latices by the use of a low molecular weight conjugated diene/carboxylic acid polymer in the form of an alkali metal salt dissolved in water as the dispersing aid or dispersing latex.
U.S. Pat. No. 4,482,657 describes mixtures of silica and synthetic polymers prepared by treating a polymer latex with a dispersion of silica and an alkyl trimethyl ammonium halide in water. Not only does the quaternary ammonium halide disrupt the blending of carbon black, the process requires the slow addition of the silica dispersion to prevent premature coagulation. Other elaborate techniques as described in U.S. Pat. No. 3,907,734 where a partitioning agent in the form of a blend of precipitated silica and hydrophobic fumed silica are incorporated into a concentrated polymer latex have been suggested. The silica adsorbs the water, and the resulting solid mixture is dried with removal of the hydrophobic fumed silica to form a free flowing blend of polymer particles coated with precipitated silica. That process is limited to relatively small scale batch system and requires recovery and recycle of the hydrophobic fumed silica. That process fails to incorporate into the polymer the more desirable hydrophobic fumed silica.
Such processes with concentrated latex, as those skilled in the art can readily appreciate, involve complex procedures not only blending the silica with the polymer latex, but also in effecting its recovery when excess silica or carbon black must be employed. Another limitation of such processes is that recovery of the filled polymer directly from the latex stage without filtration and like treatment steps used to remove byproducts from the emulsion polymerization can have deleterious effects on the end use properties of the polymer thus recovered. Such problems can be seen in French Patent 2,558,008 and French Patent 2,558,874. In the first, the addition to a rubber latex of precipitated silica effects coagulation of the rubber polymer. In the second, a stable latex of derivatized silica and a carboxylated butadiene rubber is prepared to add to natural or synthetic elastomer latices. The derivatization of the silica is accomplished by treatment with polyamines, polyethylene amines or nonionic polyoxyethylene. Such free agents are wholly incompatible with typical coagulation techniques used in the recovery of the emulsion process polymers.